A multicentre European study of Pseudomonas aeruginosa serotype O12 isolates with a common antibiotic resistance pattern was conducted. Resistance to beta-lactams and aminoglycosides was observed in 24 of the 25 isolates, as often reported in Europe, and all 25 isolates were significantly more susceptible to fosfomycin than 189 isolates of other serotypes (72% vs. 13.2%). The mutational frequency of serotype O12 was similar to that of other serotypes and thus could not explain the susceptibility to fosfomycin. As a number of epidemiological studies using various methods, especially ribotyping with EcoRI, have shown that most strains are similar, it has been suggested that a single strain of this serotype is widespread. However, in this study ribotyping with EcoRI and PvuII distinguished seven clones among 24 ticarcillin resistant serotype O12 isolates, although one ribotype predominated (67%). Thus the hypothesis of spread of one clone across Europe cannot explain the common resistance phenotype observed in different clones of serotype O12. Resistance of beta-lactams and aminoglycosides might be explained by greater receptiveness for transposable resistance mechanisms, and susceptibility to fosfomycin by increased permeability of the outer membrane.